Long-Term Evolution (LTE) is the trademark of the Third Generation Partnership Project (3GPP) and is aimed to become the next generation mobile network technology. Since the development of LTE, different transmission modes exist which have been developed to cover different environment scenarios. (See 3GPP 36.211, “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Channels and Modulation (Release 8)”, Sophia Antipolis, 2009, and 3GPP 36.213, “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical layer procedures (Release 8)”, Sophia Antipolis, 2009, which are hereby incorporated by reference. The transmission modes can be divided into two groups. The first group provides feedback in terms of channel state information (CSI) from a user equipment (UE) to the evolved NodeB (eNodeB) and is therefore named “closed-loop” transmission. The other group provides no feedback and is named “open-loop” transmission. Furthermore, LTE provides transmission modes, where the eNodeB groups a number of UEs and serves them using the same time-frequency blocks. These transmission modes are called multi-user Multiple Input Multiple Output (MU-MIMO).
The eNodeB performs digital beamforming using a finite set of precoding vectors depending on the number of transmit antenna ports. This approach is particularly advantageous in a dense cell with rare resources. Instead of waiting for free resource blocks, the eNodeB may group UEs and allocate the same resource blocks to prevent waiting periods and therefore increases spectral efficiency. The main drawback of this approach is the increase of mutual interference at each of these two UEs. The signal to both UEs is precoded, summed up and transmitted by the eNodeB, so each user equipment is responsible to filter out its own signal.
If the eNodeB does not find an appropriate candidate for grouping, only a single UE is served by the eNodeB. The UE is unaware if there is another co-scheduled UE, as far as the eNodeB does not provide this information to save transmission bandwidth. In general, the eNodeB does not provide any explicit information on whether the transmission is either a single user-(SU) or multi user-(MU) MIMO transmission. UEs operating in MU-MIMO scenarios may always choose an MMSE (Minimum-Mean-Square-Error) receive filter, although there may be no co-scheduled UE available. Although this receive filter does not fit best, it provides acceptable results, but performs worse when no co-scheduled UE is available.
Therefore, it is highly important to develop receiver algorithms that can determine the presence of a co-scheduled UE at the UE from the received data.